Aromatic uretdione-diisocyanates ("dimeric" diisocyanates) and their methods of preparation are known. See, e.g., Kunststoff-Handbuch, Volume VII, Polyurethane, published by Vieweg-Hochtlen, Carl Hanser Verlag, Munich 1966. The dimerization catalysts used in the art include not only trialkylphosphines (J. Org. Chem. 8, 23 (1943)) but also aromatic-aliphatic tertiary phosphines, alkyldiarylphosphines (German Auslegeschrift No. 2,452,390), tri- or tetra-substituted pyridines (British Pat. No. 821,158), trialkylphosphite (German Auslegeschrift No. 2,349,726) and phosphorous acid tris-dialkylamides (U.S. Pat. No. 3,290,288). The dimerization reaction to form the uretdione may in some cases take place solvent-free but solvents which are inert towards isocyanate groups are in most cases used. The following solvents, for example, are described in the art: benzene, toluene, xylene, chlorobenzene, nitrobenzene, acetone, methyl ethyl ketone, ethyl acetate, dioxane, tetrahydrofuran, aliphatic hydrocarbons, dimethylformamide and methylene chloride. No differentiation of the above-mentioned solvents according to their polarity or their power of dissolving the starting materials or products of the reaction is described in the literature noted above.
It is known that the dimerization of aromatic isocyanates, especially in the presence of reactive dimerization catalysts, may be accompanied by the formation of unwanted by-products, e.g. isocyanurate (see, e.g., U.S. Pat. No. 2,681,144). Dimerization is thus generally stopped when a desired dimerization stage has been reached. Alkylating agents such as cyclohexane sulphonic acid methyl ester or benzyl chloride, are used as inhibitors (stoppers) for the dimerization catalyst.
According to the literature cited above, aromatic diisocyanates in which the aromatic nucleus is substituted by groups R, e.g. alkyl groups, in one or both ortho-positions to the NCO group give rise to uretdione-diisocyanates having twice the molecular weight of the starting compound, e.g. the dimeric tolylene diisocyanate ##STR3##
This teaching is also applied to those aromatic dissocyanates in which the aromatically bound NCO group only has hydrogen atoms in the ortho-positions. The usual term frequently used in the literature for such uretdione diisocyanates, e.g. for those based on 4,4'-diphenylmethanediisocyanate, is therefore "diphenylmethanediisocyanate dimer" or 4,4'-diisocyanatodiphenylmethane-uretdione (MDI-uretdione) or dimeric MDI, and is represented by the following formula: ##STR4##
In German Auslegeschrift No. 1,445,721 (corresponding to U.S. Pat. No. 3,290,288), information was given concerning oligomeric uretdione-diisocyanate derivatives based on 4,4'-diphenylmethanediisocyanate. It is mentioned there that the dimerization of diphenylmethane diisocyanate results not only is dimeric uretdiones as end products but also in trimeric, tetrameric and pentameric uretdiones ##STR5## (where n=0 to 3). (See also U.S. Pat. No. 4,442,280.) "Oligomeric" dimers or uretdiones of MDI contain the uretdione group more than once in the molecule. Low-oligomeric products preferably contain this group on average n.ltoreq.1 times (i.e., a maximum of two uretdione groups).
The use of dimerized diphenylmethanediisocyanate is described in German Auslegeschrift No. 2,419,968 for the synthesis of high molecular weight polyurethanes containing uretdione groups by a reaction in highly polar solvents such as DMF.
With increasing degree of oligodimerization (for example, at n&gt;1, and in particular &gt;2.5), however, the reactivity towards active hydrogen containing compounds markedly falls due to the decreasing solubility of these uretdione diisocyanates. Clearly, such relatively high molecular weight oligomers are no longer suitable in practice for many polyurethane reactions, especially if the reactions are carried out solvent-free or at temperatures below the (relatively high) melting points of the uretdione diisocyanates.
The importance of the solvent for the dimerization reaction is only described in a few cases in the patent literature.
Some of the inert solvents mentioned as suitable for the dimerization of aromatic isocyanates are found to be unusable for the dimerization of diphenylmethanediisocyanate since high molecular weight MDI-dimer mixtures (n&gt;2.5) are obtained which are in most cases unsuitable for further polyurethane reactions. In very highly polar solvents (such as dimethylformamide or dimethylacetamide), the end products obtained consist almost exclusively of polydimers (n.gtoreq.2.5).
The importance of the solvent for the dimerization of 4,4'-diisocyanatodiphenylmethane is disclosed in European Pat. No. 0,071,899 (corresponding to U.S. Pat. No. 4,521,338). This reference describes that the nature or quantity of the catalyst and the controlled stopping of the catalyst at a certain degree of dimerization are important to produce the desired result. The reference also describes that diphenylmethane-uretdione diisocyanates having a very low degree of oligodimerization n (n.ltoreq.1, preferably .ltoreq.0.5) are obtained by using a mixture of apolar and polar solvents, especially if the apolar solvent has only a slight dissolving power for diphenylmethane-4,4'-diisocyanate (5 to 25% by weight, preferably 5 to 15% by weight). It is further described in the reference that the dimerization of 4,4'-diisocyanatodiphenylmethane in solution in an apolar solvent may also be carried out in admixture with a polar solvent. Due to the high dilution (high proportion of apolar solvent), however, this process is less economical. Furthermore, the low molecular weight diphenylmethane-uretdione diisocyanates formed still contain monomeric diphenylmethanediisocyanate starting components in a proportion of up to 20% by weight (depending on the quantity of solvent used), in most cases from 2 to 10% by weight based on the dimer (I).